The present invention relates to a phased-array antenna consisting of multiple radiating elements where at least one of the radiating antenna elements is rotatable on its axis. More particularly, the invention relates to an antenna array of multi-filar helical antenna elements which are individually rotated to control the phase of the antenna array.and thereby steer the beam of the antenna system.
In satellite communications, airborne antennas serve multiple purposes such as providing voice communications to the cockpit and the cabin of the aircraft, data and Internet services, and more recently, real-time video streaming. Such antennas are required to have low noise temperature and a compact size to minimize the drag on the aircraft. For certain applications, a small footprint is required for the antenna system.
For transmission and reception over various global networks, an aircraft requires an antenna beam that can track satellites effectively. The antenna beam must be capable of being directed towards various satellites regardless of aircraft orientation. Various types of high gain antennas such as a mechanically steered antenna, a switched beam antenna, or a phased-array antenna, can provide this functionality.
Mechanically steered antennas are usually mounted inside the tail section of an aircraft where the height limitations on the antennas are not as important. However, gaining access to the tail of a larger aircraft is often quite difficult, due in part to the height of the tail and also in part to the weight of the radome where the antenna is placed. Such constraints limit the access to antennas located inside the tail of an aircraft.
Another option available for antenna beam steering is the use of switched beam antennas. Unfortunately, switched antenna beams suffer from low gain at the beam cross-over points and a large phase discontinuity when beams are switched.
The phased-array antenna, on the other hand, offers distinct advantages. The phased-array antenna consists of a group of radiating elements which are distributed and oriented on a planar surface in various spatial configurations. Depending on the amplitude and phase excitation of each radiating element, the resulting antenna beam can be controlled. Phased-array antennas, which may have a low profile design, are normally mounted on top of the fuselage of an aircraft. Conventional phased-array antennas usually experience losses in gain and an undesirable increase in sidelobe levels, resulting from the quantization of the phase settings of the antenna elements. Typically, these conventional phased-array antennas employ switched phase shifters. Such conventional switched phase shifters usually have a minimum phase step size of approximately 22.5 or 45xc2x0. The 22.5xc2x0 step size is a considerable phase shift and as a result a remnant phase discontinuity is introduced when the beam is scanned from one position to the next and the resolution in angular pointing is finite. It is highly desirable to provide an antenna array system where the phase settings are not quantized and where the phase error for each antenna element can be maintained within a fraction of a degree.
Conventional phased-array antennas typically use positive-intrinsic-negative (PIN) diode phase shifters. In a phased-array antenna, most antenna radiating elements have their own dedicated phase shifter. The phase shifter permits the radiated power for each antenna element to be adjusted over a +/xe2x88x92180 degree range relative to the other antenna elements. Unfortunately, the discrete stepping of the phase angle results in some degree of phase error relative to the ideal phase of the antenna element. This phase error results in increased sidelobe levels and reduced gain, and affects the direction of the antenna beam. The spreading and squinting of the antenna beam and their attendant gain losses are significant hindrances in airborne applications. Furthermore, the dissipative loss of the phase shifter""s reduces the antenna gain and also increases the antenna noise level, usually referred to as the antenna noise temperature.
The present invention seeks to overcome the above shortcomings by providing a phased-array antenna system with a narrow beam. Furthermore, the antenna beam coverage is increased through use of multi-filar helix antenna elements which are each rotatable on their axes in order to change the angular orientation of the antenna beam. The present invention does not require the use of phase shifters to achieve phase control, thereby minimizing the phase errors and gain losses introduced by such shifters.
The present invention seeks to provide a phased-array antenna whose main beam can be positioned (scanned) substantially anywhere within a full hemispherical coverage. The phased-array antenna consists of a plurality of radiating multi-filar helix antenna elements, each of which is independently rotatable on its helix axis relative to the antenna base plane. The rotation of each radiating element substantially decreases the noise temperature, the passive intermodulation of signals, and the sidelobe levels relative to an antenna which uses PIN diode phase shifters. A change in the radiated signal phase of each element in the antenna array is achieved by the mechanical rotation of the multi-filar helical antenna element on its axis. Mechanical rotation is provided by a drive system or multiple drive systems coupled to at least one radiating antenna element. The drive system rotates the at least one radiating element on its axis by various amounts. The amount of rotation is directly proportional to the amount of phase shift required of that particular multi-filar helical antenna element relative to the other antenna elements.
In an alternative embodiment, there is provided a phased-array antenna consisting of a plurality of radiating multi-filar helix antenna elements, together with a plurality of drive systems with each drive system coupled to a multi-filar helix element. The amount of rotation required for each helix element is determined by the position of each element relative to the other elements in the array and the desired antenna beam angle. Thus, the amount of rotation for each element provided by each of the drive systems varies according to the design specification.
In a first aspect, the present invention provides an antenna system including:
(a) a base;
(b) at least one stationary multi-filar helical antenna element, the or each stationary multi-filar helical antenna element having a longitudinal axis, the or each stationary multi-filar helical antenna element having a first feed network coupled to a base end of the stationary antenna element, the first feed network having a feed end located at the base end of.a corresponding stationary antenna element;
(c) at least one rotatable multi-filar helical antenna element, the or each rotatable multi-filar helical antenna element having a longitudinal.axis, the or each rotatable multi-filar helical antenna element having a second feed network coupled to a base end of the antenna element, each second feed network having a feed end located at the.base end of a corresponding rotatable antenna element, wherein the or at least one rotatable multi-filar helical antenna elements is rotatable on its longitudinal axis;
(d) at least one drive system connected to a corresponding one of the at least one rotatable multi-filar helical antenna elements, the at least one drive system being capable of rotating the corresponding at least one of the rotatable helical antenna elements about its longitudinal axis;
(e) a power, splitting and combining means for feeding input power to each feed network belonging to each of the at least one stationary multi-filar helical antenna elements and the at least one rotatable multi-filar helical antenna element;
wherein the or each stationary antenna element is supported by the base at the base end of the stationary antenna element,
wherein the or each rotatable antenna element is supported by the base at the base end of the rotatable antenna element, and
wherein the power splitting and combining means feeds input power to the feed end of each first feed network and the feed end.of.each second feed network.
In a second aspect, the present invention provides an antenna system including:
(a) a base;
(b) at least one stationary monofilar helical antenna element, the breach stationary monofilar helical antenna element having a longitudinal axis, the or each stationary monofilar helical antenna element having a first feed network coupled to a base end of the stationary antenna element, the first feed network having a feed end located at the base end of a corresponding stationary antenna element;
(c) at least one rotatable monofilar helical antenna element, the or each rotatable monofilar helical antenna element having a longitudinal axis, the or each rotatable monofilar helical antenna element having a second feed network coupled to a base end of the antenna element, each second feed network having a feed end located at the base end of a corresponding rotatable antenna element, wherein the or at least one rotatable: monofilar helical antenna elements is rotatable on its longitudinal axis;
(d) at least one drive system connected to a corresponding of the at least one rotatable monofilar helical antenna elements, the at least one drive system being capable of rotating the corresponding at least one of the rotatable helical antenna elements about its longitudinal axis;
(e) a power splitting and combining means for feeding input power to each feed network belonging to each of the at least one stationary monofilar helical antenna elements and the at least one rotatable monofilar helical antenna element;
wherein the or each stationary antenna element is supported by the base at the base end of the stationary antenna element,
wherein the or each rotatable antenna element is supported by the base at the base end of the rotatable antenna element, and
wherein the power splitting and combining means feeds input power to the feed end of each first feed network and the feed end of each second feed network.